> I have questions about this, and maybe you can help.
> From my reading it generally appears that most plant physiologist believe
> the cell walls "loosen" prior to elongating. I have trouble with
> "loosening" as a word to discribe what happens.
>> In my understanding cells will only expand prior to secondary cell wall
> deposition, and the primary cell wall is in a more or less continuous state
> of deposition prior to 2nd cell wall formation.
>> Therefore, since the cell wall is actively growing one does not need to
> hypothesize a cell wall "loosening agent" to allow for expansion... the
> cell wall is loose anyway, and it eventually loses this "looseness" as it
> matures.
>>Cellulose microfibrils are tightly hydrogen-bonded to xyloglucan
molecules, which may bridge from one microfibril to the next. Cells
generally expand perpendicular to the major axis of microfibril
orientation, and so expansion requires either or both (1) breakage of
the xyloglucan polymers (the XET activity theory), or (2) interuption
of hydrogen bonding (Cosgrove's expansin theory). "Loosening" refers
to the breakage of bonds between adjacent microfibrils. If this is
difficult to picture, see Cosgrove's review (1993. Plant Physiol.
102, 1-6).
Most cells do not form a secondary wall, and cease elongating never-
the-less. Stem growth is a good example. This may be due to cross-
linking of cell wall proteins (see Bradley et al., 1992. Cell 70, 21-
30).
I am forwarding this message to plant-biology in hopes of further
discussion.
Kate VandenBosch